1. Technical Field
Embodiments of this disclosure generally relate to a fixing device employing a heating method and to an electrophotographic image forming apparatus, such as a copier, a printer, or a facsimile machine, incorporating the fixing device.
2. Related Art
Image forming apparatuses, such as copiers, facsimile machines, or printers usually form a toner image on an image carrier according to image data. The toner image is transferred from the image carrier onto a recording medium such as a sheet of paper or an overhead projector (OHP) sheet. The recording medium carrying the toner image is then conveyed to a fixing device, in which the toner image is fixed onto the recording medium under heat and pressure.
Fixing devices employing a heat-roller method usually include a fixing roller and a pressing roller. The fixing roller is heated by a heat source such as a halogen heater or an induction heating coil. The fixing roller and the pressing roller press against each other to form an area of contact herein called a nip, to which a recording medium carrying a toner image is conveyed. Toner included in the toner image is fused under heat and pressure in the nip. Thus, the toner image is fixed onto the recording medium. Such fixing devices are widely used for safety and adaptability to high-speed machines.
However, it takes a few minutes for the fixing roller, typically having a metal core and a large heat capacity, to reach a predetermined fixing temperature. Hence, the fixing roller is maintained at a predetermined temperature during standby time, resulting in relatively large energy consumption.
By contrast, fixing devices employing a belt or film method are frequently used for energy efficiency. Several energy-efficient fixing techniques employed with such fixing devices have been proposed, such as those externally heating a thermal insulating roller or selectively heating an imaged area according to the image data.
For example, JP-H06-095540-A discloses a fixing device employing the film method, in which a pressing roller and a planar heater that contact a thin, cylindrical film having thermal resistance sandwiches the film and a recording material so that the film and the recording material adhere to each other, thereby heating the recording material. Because the film has a thickness of only about 100 μm, in actuality the fixing device can be warmed up simply by increasing the temperature of the low heat-capacity planar heater. Accordingly, the warm-up time can be shortened and the preheating power can be reduced.
In addition, JP-H06-095540-A discloses a technique whereby the temperature of the heater and the heating areas are changed based on an image formed on the recording material to reduce energy supply to a blank area (i.e., a portion of an image formation area without an image), thereby enhancing energy efficiency.
JP-2005-181946-A discloses a technique whereby the temperature of a thermal heater is measured for each heating element to supply appropriate heat, thereby heating only a portion where toner exists on a surface of a sheet, taking into account the surrounding temperature.
JP-2001-343860 employs a fixing method to externally heat a roller. The external heating allows toner to be fused by heat accumulated on and around a fixing roller. Accordingly, such an external heating method realizes a shorter warm-up time and a higher energy efficiency than an internal heating method to heat an entire fixing roller. As in JP-H06-095540-A and JP-2005-181946-A, JP-2001-343860 discloses that imaged areas are selectively heated and that a second target temperature is provided which is lower than a target fixing temperature.
Typical fixing devices are supplied with a maximum energy sufficient to fix an image formed on an entire surface of a recording material.
However, when an imaged area is selectively heated, electric power is supplied before that imaged area enters the fixing nip. In other words, a preliminary heating area is provided, taking into account a predetermined time taken to warm up a heating member including a heat generator. The preliminary heating area is provided in a blank area that does not bear an unfixed toner image. Hence, the preliminary heating area is preferably as small as possible.